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dounmount(9): temporarily enable recursion for the covered vnode lock

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For some complex nullfs mount configurations, it is possible to get the
covered vnode lock for the mount shared with some inside-mount vnode
lock.  Then at unmount time, vflush() would recurse on the covered vnode
lock when reclaiming the vnode.

Work around it, by temprorarily allowing recursion on the covered vnode
lock.  Disable recursion after the unmount if it was not enabled before.

PR:	297174
Reviewed by:	jah
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D58567
This commit is contained in:
Konstantin Belousov committed 2026-08-07 12:09:50 +03:00
1 parent 1d97ad676d
commit 9f5c4ef328
1 file changed
+26 -3
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sys/kern/vfs_mount.c
+26 -3
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@@ -1880,7 +1880,8 @@ vfs_check_usecounts(struct mount *mp)
}
static void
dounmount_cleanup(struct mount *mp, struct vnode *coveredvp, int mntkflags)
dounmount_cleanup(struct mount *mp, struct vnode *coveredvp, int mntkflags,
bool disablerec)
{
mtx_assert(MNT_MTX(mp), MA_OWNED);
@@ -1892,6 +1893,8 @@ dounmount_cleanup(struct mount *mp, struct vnode *coveredvp, int mntkflags)
vfs_op_exit_locked(mp);
MNT_IUNLOCK(mp);
if (coveredvp != NULL) {
if (disablerec)
VN_LOCK_DREC(coveredvp);
VOP_UNLOCK(coveredvp);
vdrop(coveredvp);
}
@@ -2193,6 +2196,7 @@ dounmount(struct mount *mp, uint64_t flags, struct thread *td)
uint64_t async_flag;
int mnt_gen_r;
unsigned int retries;
bool coveredrec;
KASSERT((flags & MNT_DEFERRED) == 0 ||
(flags & (MNT_RECURSE | MNT_FORCE)) == (MNT_RECURSE | MNT_FORCE),
@@ -2301,6 +2305,7 @@ dounmount(struct mount *mp, uint64_t flags, struct thread *td)
if ((flags & MNT_DEFERRED) != 0)
vfs_ref(mp);
coveredrec = false;
if ((coveredvp = mp->mnt_vnodecovered) != NULL) {
mnt_gen_r = mp->mnt_gen;
VI_LOCK(coveredvp);
@@ -2317,6 +2322,19 @@ dounmount(struct mount *mp, uint64_t flags, struct thread *td)
vfs_rel(mp);
return (EBUSY);
}
/*
* For some complex nullfs mount configurations, it is
* possible to get the covered vnode lock for the
* mount shared with some inside-mount vnode lock.
* Then at unmount time, vflush() would recurse on the
* covered vnode lock when reclaiming the vnode.
*
* To work around it, temprorarily allow recursion for
* the covered vnode lock.
*/
coveredrec = VN_LOCK_CANREC(coveredvp);
VN_LOCK_AREC(coveredvp);
}
vfs_op_enter(mp);
@@ -2326,7 +2344,7 @@ dounmount(struct mount *mp, uint64_t flags, struct thread *td)
if ((mp->mnt_kern_flag & MNTK_UNMOUNT) != 0 ||
(mp->mnt_flag & MNT_UPDATE) != 0 ||
!TAILQ_EMPTY(&mp->mnt_uppers)) {
dounmount_cleanup(mp, coveredvp, 0);
dounmount_cleanup(mp, coveredvp, 0, !coveredrec);
return (EBUSY);
}
mp->mnt_kern_flag |= MNTK_UNMOUNT;
@@ -2339,7 +2357,8 @@ dounmount(struct mount *mp, uint64_t flags, struct thread *td)
MNT_ILOCK(mp);
if (error != 0) {
vn_seqc_write_end(coveredvp);
dounmount_cleanup(mp, coveredvp, MNTK_UNMOUNT);
dounmount_cleanup(mp, coveredvp, MNTK_UNMOUNT,
!coveredrec);
if (rootvp != NULL) {
vn_seqc_write_end(rootvp);
vrele(rootvp);
@@ -2425,6 +2444,8 @@ dounmount(struct mount *mp, uint64_t flags, struct thread *td)
MNT_IUNLOCK(mp);
if (coveredvp) {
vn_seqc_write_end(coveredvp);
if (!coveredrec)
VN_LOCK_DREC(coveredvp);
VOP_UNLOCK(coveredvp);
vdrop(coveredvp);
}
@@ -2445,6 +2466,8 @@ dounmount(struct mount *mp, uint64_t flags, struct thread *td)
coveredvp->v_mountedhere = NULL;
vn_seqc_write_end_locked(coveredvp);
VI_UNLOCK(coveredvp);
if (!coveredrec)
VN_LOCK_DREC(coveredvp);
VOP_UNLOCK(coveredvp);
vdrop(coveredvp);
}